(a) Field of the Invention
The present invention relates to a method for preparing soluble dietary fiber from corn hulls.
(b) Description of the Related Arts
In general, dietary fiber can be defined as “all the components of a plant that are resistant to digestion by human digestive enzymes.”
Dietary fiber can be classified into a water-insoluble group and a water-soluble group. It has been known that soluble dietary fiber has a good effect on suppression of increase of serum cholesterol level. The mechanism of soluble dietary fiber to improve lipid metabolism is supported by the functions that soluble dietary fiber inhibits lipid absorption in the digestive tract and accelerates excretion of bile acid, and that the short chain fatty acids, in particular propionic acid produced from fermentation of the fiber in the colon, suppresses the synthesis of cholesterol. Well-known insoluble dietary fibers include cellulose, lignin, and the like, and soluble dietary fibers include Guar gum, Arabic gum, pectin, hemicellulose, and the like. In addition, indigestible components that are contained in cell walls and cell contents of plants such as grains have been called crude fiber, and they include hulls of grains such as rice, corn, and beans.
Corn hulls, produced from wet milling in the preparation process of starch, can be used as a good source of dietary fiber, because the corn hulls contain a lot of dietary fiber, and in particular more hemicellulose than hulls of other grains. Hemicellulose is a polysaccharide that consists of complicatedly connected monomers, and that is a component of cell walls of plants except for cellulose and pectin. Hemicelluloses are divided into two groups: water-insoluble hemicellulose A, and water-soluble hemicellulose B. Soluble hemicellulose B includes a main component of arabinoxylan which consists of xylose and arabinose. Arabinoxylan has a beta-1,4-linkage that cannot be hydrolyzed by human digestive enzymes, and thus can be a good dietary fiber.
There are many suggestions on preparation methods for dietary fiber from grain hulls.
Japanese Laid-Open Patent Publication Pyung 1-242540 described a process for preparing dietary fiber powder. Corn hulls were treated with glucoamylase for 24 to 40 hours to remove starch, extracted at room temperature for 18 hours after adding 0.5N NaOH solution (2%), and centrifuged. The extract was then treated with trichloroacetic acid and centrifuged again to precipitate protein.
The resultant solution was contained in a cellophane tube for dialysis in water for 3 days, and was then left after adding ethanol to obtain precipitates, which were subsequently recovered (final yield=13%).
Japanese Laid-Open Patent Publication Pyung 3-209331 disclosed a preparation method of dietary fiber powder. Corn hulls obtained from wet milling in a preparation process of corn starch were mixed in a homogenizer with the addition of water, and then filtered with a sieve. The resultant was extracted with a NaOH or Ca(OH)2 solution, centrifuged, and neutralized to pH 7. The neutralized solution was dried in a drum dryer, or it was treated with alkaline xylase, purified in several processes, and then dried in a spray dryer to produce powder.
Japanese Patent Publication Pyung 6-11764 disclosed a method of to preparing soluble hemicellulose. Defatted rice bran was treated with heat-resistant amylase in hot water to remove starch, and was then extracted under an alkaline condition of pH 10 or more, or an acidic condition of pH 3 or more, to produce a solution containing a soluble hemicellulose B fraction. Insoluble components were removed by neutralizing the resultant solution and performing ultra-filtration. The resultant was purified to produce soluble hemicellulose (final yield=3.5˜8%).
WO 98-40413 disclosed a preparing method for corn fiber gum with an improved color and low viscosity. Specifically, the corn hulls were destarched with heat-resistant amylase, extracted with a hydrogen peroxide, NaOH, and a Ca(OH)2 solution, and then ultra-filtrated to remove insoluble components such as a hemicellulose A fraction. The resultant solution was concentrated and dried in a spray dryer. Otherwise the solution was precipitated with alcohol after concentration.
The preparing methods for soluble dietary fiber from grain hulls as mentioned above have many disadvantages, as follows. The fine fiber is removed in the removing process of starch and protein, thereby lowering the yield of fiber. Because Ca(OH)2 with low water-solubility is used in the alkaline extracting process, the content of ash is high in the final sample. Thus, many purification processes are required, and the extraction yield is decreased. In addition, due to centrifugation after alkaline extraction, the extract solution cannot be recovered maximally. Thus, the final yield of soluble fiber decreases, and the maximal recovery rate according to prior arts is only 13%. Use of organic solvent such as alcohol makes the preparation process complicated and the industrialization of the process difficult.